Lipoproteins are globular, micelle-like particles that consist of a non-polar core of acylglycerols and cholesteryl esters surrounded by an amphiphilic coating of protein, phospholipid and cholesterol. Lipoproteins have been classified into five broad categories on the basis of their functional and physical properties: chylomicrons, which transport dietary lipids from intestine to tissues; very low density lipoproteins (VLDL); intermediate density lipoproteins (IDL); low density lipoproteins (LDL); all of which transport triacylglycerols and cholesterol from the liver to tissues; and high density lipoproteins (HDL), which transport endogenous cholesterol from tissues to the liver.
Lipoprotein particles undergo continuous metabolic processing and have variable properties and compositions. Lipoprotein densities increase without decreasing particle diameter because the density of their outer coatings is less than that of the inner core. The protein components of lipoproteins are known as apoliproteins. At least nine apolipoproteins are distributed in significant amounts among the various human lipoproteins.
Apolipoprotein B (also known as ApoB, apolipoprotein B-100; ApoB-100, apolipoprotein B-48; ApoB-48 and Ag(x) antigen), is a large glycoprotein that serves an indispensable role in the assembly and secretion of lipids and in the transport and receptor-mediated uptake and delivery of distinct classes of lipoproteins. The importance of apolipoprotein B spans a variety of functions, from the absorption and processing of dietary lipids to the regulation of circulating lipoprotein levels (Davidson and Shelness, Annu. Rev. Nutr., 2000, 20, 169-193). This latter property underlies its relevance in terms of atherosclerosis susceptibility, which is highly correlated with the ambient concentration of apolipoprotein B-containing lipoproteins (Davidson and Shelness, Annu. Rev. Nutr., 2000, 20, 169-193).
Two forms of apolipoprotein B exist in mammals. ApoB-100 represents the full-length protein containing 4536 amino acid residues synthesized exclusively in the human liver (Davidson and Shelness, Annu. Rev. Nutr., 2000, 20, 169-193). A truncated form known as ApoB-48 is colinear with the amino terminal 2152 residues and is synthesized in the small intestine of all mammals (Davidson and Shelness, Annu. Rev. Nutr., 2000, 20, 169-193).
ApoB-100 is the major protein component of LDL and contains the domain required for interaction of this lipoprotein species with the LDL receptor. In addition, ApoB-100 contains an unpaired cysteine residue which mediates an interaction with apolipoprotein(a) and generates another distinct atherogenic lipoprotein called Lp(a) (Davidson and Shelness, Annu. Rev. Nutr., 2000, 20, 169-193).
In humans, ApoB-48 circulates in association with chylomicrons and chylomicron remnants and these particles are cleared by a distinct receptor known as the LDL-receptor-related protein (Davidson and Shelness, Annu. Rev. Nutr., 2000, 20, 169-193). ApoB-48 can be viewed as a crucial adaptation by which dietary lipid is delivered from the small intestine to the liver, while ApoB-100 participates in the transport and delivery of endogenous plasma cholesterol (Davidson and Shelness, Annu. Rev. Nutr., 2000, 20, 169-193).
The basis by which the common structural gene for apolipoprotein B produces two distinct protein isoforms is a process known as RNA editing. A site specific cytosine-to-uracil editing reaction produces a UAA stop codon and translational termination of apolipoprotein B to produce ApoB-48 (Davidson and Shelness, Annu. Rev. Nutr., 2000, 20, 169-193).
Apolipoprotein B was cloned in 1985 (Law et al., Proc. Natl. Acad. Sci. U.S.A., 1985, 82, 8340-8344) and mapped to chromosome 2p23-2p24 in 1986 (Deeb et al., Proc. Natl. Acad. Sci. U.S.A., 1986, 83, 419-422).
Disclosed and claimed in U.S. Pat. No. 5,786,206 are methods and compositions for determining the level of low density lipoproteins (LDL) in plasma which include isolated DNA sequences encoding epitope regions of apolipoprotein B-100 (Smith et al., 1998).
Transgenic mice expressing human apolipoprotein B and fed a high-fat diet were found to develop high plasma cholesterol levels and displayed an 11-fold increase in atherosclerotic lesions over non-transgenic littermates (Kim and Young, J. Lipid Res., 1998, 39, 703-723; Nishina et al., J. Lipid Res., 1990, 31, 859-869).
In addition, transgenic mice expressing truncated forms of human apolipoprotein B have been employed to identify the carboxyl-terminal structural features of ApoB-100 that are required for interactions with apolipoprotein(a) to generate the Lp(a) lipoprotein particle and to investigate structural features of the LDL receptor-binding region of ApoB-100 (Kim and Young, J. Lipid Res., 1998, 39, 703-723; McCormick et al., J. Biol. Chem., 1997, 272, 23616-23622).
Apolipoprotein B knockout mice (bearing disruptions of both ApoB-100 and ApoB-48) have been generated which are protected from developing hypercholesterolemia when fed a high-fat diet (Farese et al., Proc. Natl. Acad. Sci. U.S.A., 1995, 92, 1774-1778; Kim and Young, J. Lipid Res., 1998, 39, 703-723). The incidence of atherosclerosis has been investigated in mice expressing exclusively ApoB-100 or ApoB-48 and susceptibility to atherosclerosis was found to be dependent on total cholesterol levels. Whether the mice synthesized ApoB-100 or ApoB-48 did not affect the extent of the atherosclerosis, indicating that there is probably no major difference in the intrinsic atherogenicity of ApoB-100 versus ApoB-48 (Kim and Young, J. Lipid Res., 1998, 39, 703-723; Veniant et al., J. Clin. Invest., 1997, 100, 180-188).
Elevated plasma levels of the ApoB-100-containing lipoprotein Lp(a) are associated with increased risk for atherosclerosis and its manifestations, which may include hypercholesterolemia (Seed et al., N. Engl. J. Med., 1990, 322, 1494-1499), myocardial infarction (Sandkamp et al., Clin. Chem., 1990, 36, 20-23), and thrombosis (Nowak-Gottl et al., Pediatrics, 1997, 99, E11).
The plasma concentration of Lp(a) is strongly influenced by heritable factors and is refractory to most drug and dietary manipulation (Katan and Beynen, Am. J. Epidemiol., 1987, 125, 387-399; Vessby et al., Atherosclerosis, 1982, 44, 61-71). Pharmacologic therapy of elevated Lp(a) levels has been only modestly successful and apheresis remains the most effective therapeutic modality (Hajjar and Nachman, Annu. Rev. Med., 1996, 47, 423-442).
Disclosed and claimed in U.S. Pat. No. 6,156,315 and the corresponding PCT publication WO 99/18986 is a method for inhibiting the binding of LDL to blood vessel matrix in a subject, comprising administering to the subject an effective amount of an antibody or a fragment thereof, which is capable of binding to the amino-terminal region of apolipoprotein B, thereby inhibiting the binding of low density lipoprotein to blood vessel matrix (Goldberg and Pillarisetti, 2000; Goldberg and Pillarisetti, 1999).
Disclosed and claimed in U.S. Pat. No. 6,096,516 are vectors containing cDNA encoding murine recombinant antibodies which bind to human ApoB-100 for the purpose of for diagnosis and treatment of cardiovascular diseases (Kwak et al., 2000).
Disclosed and claimed in PCT publication EP 911344 is a monoclonal antibody which specifically binds to ApoB-48 and does not specifically bind to ApoB-100, which is useful for diagnosis and therapy of hyperlipidemia and arterial sclerosis (Uchida and Kurano, 1998).
Disclosed and claimed in PCT publication WO 01/30354 are methods of treating a patient with a cardiovascular disorder, comprising administering a therapeutically effective amount of a compound to said patient, wherein said compound acts for a period of time to lower plasma concentrations of apolipoprotein B or apolipoprotein B-containing lipoproteins by stimulating a pathway for apolipoprotein B degradation (Fisher and Williams, 2001).
Disclosed and claimed in U.S. Pat. No. 5,220,006 is a cloned cis-acting DNA sequence that mediates the suppression of atherogenic apolipoprotein B (Ross et al., 1993).
Disclosed and claimed in PCT publication WO 01/12789 is a ribozyme which cleaves ApoB-100 mRNA specifically at position 6679 (Chan et al., 2001).
To date, strategies aimed at inhibiting apolipoprotein B function have been limited to Lp(a) apheresis, antibodies, antibody fragments and ribozymes. However, with the exception of Lp(a) apheresis, these investigative strategies are untested as therapeutic protocols. Consequently, there remains a long felt need for additional agents capable of effectively inhibiting apolipoprotein B function.
Antisense technology is emerging as an effective means of reducing the expression of specific gene products and may therefore prove to be uniquely useful in a number of therapeutic, diagnostic and research applications involving modulation of apolipoprotein B expression.
The present invention provides compositions and methods for modulating apolipoprotein B expression, including inhibition of the alternative isoform of apolipoprotein B, ApoB-48.